Authentication of an electromagnetic terminal-transponder couple by the terminal

ABSTRACT

A method of authentication, by a terminal generating a magnetic field, of a transponder located in this field, wherein: first data, relative to the current in an oscillating circuit of the terminal, measured by the terminal for a first value of the resistive load of the transponder, are transmitted to the transponder; second corresponding data are evaluated by the transponder for a second value of the resistive load and are transmitted to the terminal; and said second data are compared with third corresponding data, measured by the terminal for the second value of the resistive load.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patentapplication Ser. No. 09/54351, filed on Jun. 25, 2009, entitled“AUTHENTICATION OF AN ELECTROMAGNETIC TERMINAL-TRANSPONDER COUPLE BY THETERMINAL,” which is hereby incorporated by reference to the maximumextent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electronic systems, and morespecifically to systems using electromagnetic transponders, that is,transceivers capable of being interrogated in a contactless and wirelessmanner by a read and/or write terminal.

2. Discussion of the Related Art

Many communication systems are based on a modulation of anelectromagnetic field generated by a terminal. They range from thesimplest electronic tag used as a theft-prevention device to morecomplex systems where a transponder intended to communicate with theterminal when it is in its field, is equipped with calculation functions(electronic purse, for example) or data processing functions.

Electromagnetic transponder systems are based on the use of oscillatingcircuits comprising a winding forming an antenna, on the transponderside and on the terminal side. Such circuits are intended to be coupledby a near magnetic field when the transponder enters the field of theterminal. The oscillating circuits of the terminal and of thetransponder are generally tuned to the same frequency corresponding tothe excitation frequency of the oscillating circuit of the terminal.

In most cases, transponders have no autonomous power supply and extractthe power supply necessary to their circuits from the high-frequencyfield radiated by the antenna of the terminal.

When a transponder needs to communicate with a terminal, the transpondermay have to authenticate the terminal before accepting a data exchange.For example, in applications where the transponder is used as a paymentmeans (be it in money or units of account), it may reserve payments tocertain terminals. According to another example, a transponder of chipcard type, associated with electronic equipment (for example, a personaldigital assistant or a cell phone) identifies or authenticates a user ina communication with other electronic equipment (for example, a laptopor desktop computer).

Symmetrically, the terminal may have to authenticate the transponderbefore transmitting certain data thereto.

Currently, authentication processes use cryptography algorithms and adata exchange between the terminal and the transponder. Such processesrequire significant power- and time-intensive calculations. Further, anycryptographic process is more or less sensitive to attacks aiming atdiscovering the secret of the authentication to hack the system.

SUMMARY OF THE INVENTION

It would be desirable for a transponder to be able to authenticate aterminal with which it needs to communicate before it has to transmitdata to the terminal, and for the terminal to also be able toauthenticate the transponder.

It would also be desirable to have an authentication process independentfrom any cryptography.

It would also be desirable to have a fast, less power- andcalculation-intensive authentication process.

To achieve all or part of these objects as well as others, at least oneembodiment of the present invention provides a method of authentication,by a terminal generating a magnetic field, of a transponder located inthis field, wherein:

first data, relative to the current in an oscillating circuit of theterminal, measured by the terminal for a first value of the resistiveload of the transponder, are transmitted to the transponder;

second corresponding data are evaluated by the transponder for a secondvalue of the resistive load and is transmitted to the terminal; and

-   -   said second data are compared with third corresponding data,        measured by the terminal for the second value of the resistive        load.

According to an embodiment of the present invention, the transponderevaluates said second data based on said first data and on fourth datarelative to the level of a D.C. voltage generated by an oscillatingcircuit of the transponder, respectively measured for said first valueof the resistive load and for a second resistive load value.

According to an embodiment of the present invention, said data areratios of the current in the oscillating circuit of the terminal whileno transponder is located in its field and of this same current with thevalues of the resistive load.

According to an embodiment of the present invention, in the absence ofan authentication, the terminal sends intentionally incorrect data.

The present invention also provides a method for authenticating aterminal generating a magnetic field and a transponder which is presentin its field, wherein:

the transponder is authenticated by the terminal; and

to authenticate the terminal, the transponder exploits said first andfourth data.

According to an embodiment of the present invention, the transponder:

evaluates, based on said first and fourth data, a ratio between valuesof the current in the oscillating circuit of the terminal; and

compares this ratio with said first data.

According to an embodiment of the present invention, the transponder:

evaluates, based on said first and fourth data, a value of said voltage;and

compares this evaluated value with the measured value.

According to an embodiment of the present invention, in the absence ofan authentication by the transponder, said transponder sendsintentionally incorrect data.

At least one embodiment of the present invention also provides anelectromagnetic transponder comprising:

an oscillating circuit upstream of a rectifying circuit capable ofproviding a D.C. voltage when the transponder is in the magnetic fieldof a terminal; and

at least one processing unit capable of implementing the authenticationmethod.

At least one embodiment of the present invention also provides aterminal capable of generating an electromagnetic field for atransponder, comprising means capable of implementing the authenticationmethod.

The foregoing objects, features, and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a very simplified representation of a transponder system ofthe type to which the present invention applies as an example;

FIG. 2 is a simplified block diagram of a terminal and of a transponderof an electromagnetic transponder communication system;

FIG. 3 is a functional block diagram illustrating an embodiment of themethod of authentication of a terminal by a transponder;

FIG. 4 illustrates a variation of the embodiment of FIG. 3; and

FIG. 5 is a block diagram of an embodiment of a transponder capable ofauthenticating a terminal.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings. For clarity, only those steps and elementswhich are useful to the understanding of the present invention have beenshown and will be described. In particular, the coding and themodulation of the communications between the transponder and theterminal have not been detailed, the present invention being compatiblewith any usual communication. Further, the functions that can beimplemented by a terminal or by a transponder, other than theauthentication by this transponder have not been detailed either, thepresent invention being here again compatible with any usual function ofa terminal or of a transponder.

FIG. 1 is a block diagram of an electromagnetic transpondercommunication system. A terminal 1 (TERMINAL) can communicate in nearfield (for example according to a near field communication protocol NFC)with a distant element, that is, a transponder (TRANS).

The terminal may take different forms, for example, a transport ticketvalidation terminal, an electronic passport reader, a laptop computer, amobile telecommunication device (GSM phone, PDA, etc.), an electroniccontrol unit for starting an automobile vehicle, etc.

The transponder may similarly take different forms, for example, a chipcard, an electronic transport ticket, an electronic passport, atelecommunication terminal (GSM phone, PDA, etc.), an electronic tag,etc.

FIG. 2 very schematically shows a simplified example of a terminal 1 andof a transponder 2.

Terminal 1 comprises an oscillating circuit, generally series, formed ofan inductance L1 in series with a capacitor C1 and a resistor R1. Thisseries oscillating circuit is, in the example of FIG. 2, connectedbetween an output terminal 12 of an amplifier or antenna coupler 14 anda terminal 13 at a reference voltage (generally the ground). An element15 for measuring the current in the oscillating circuit is interposed,for example, between capacitive element C1 and ground 13. Measurementelement 15 belongs to a phase regulation loop which will be describedhereafter. Amplifier 14 receives a high-frequency transmission signaloriginating from a modulator 16 (MOD) which receives a referencefrequency (signal OSC), for example, from a quartz oscillator (notshown). Modulator 16 receives, if need be, a signal Tx originating froma circuit 11 for controlling and exploiting the transmissions. Circuit11 is generally provided with a control and data processingmicroprocessor, communicating with different input/output circuits(keyboard, display, element of exchange with a server, etc.) and/orprocessing circuits, not shown. The elements of terminal 1 most oftendraw the power necessary to their operation from a supply circuit (notshown) connected, for example, to the power line distribution system(mains) or to a battery (for example, that of an automobile vehicle orof a portable telephone or computer). Modulator 16 provides ahigh-frequency carrier (for example, at 13.56 MHz) to series oscillatingcircuit L1-C1 which generates a magnetic field.

Capacitive element C1 is, for example, a variable-capacitance elementcontrollable by a signal CTRL. This element takes part in the phaseregulation of current I1 in antenna L1 with respect to a referencesignal. This regulation is a regulation of the high-frequency signal,that is, of the signal of the carrier corresponding to the signalprovided to amplifier 14 in the absence of data Tx to be transmitted.The regulation is performed by varying capacitance C1 of the oscillatingcircuit of the terminal to maintain the current in the antenna inconstant phase relationship with a reference signal. This referencesignal for example corresponds to signal OSC provided to modulator 14.Signal CTRL originates from a circuit 17 (COMP) having the function ofdetecting the phase interval with respect to the reference signal and ofaccordingly modifying the capacitance of element C1. The comparatorreceives data MES about current I1 in the oscillating circuit detectedby measurement element 15 (for example, an intensity transformer or aresistor).

A transponder 2, capable of cooperating with terminal 1, comprises anoscillating circuit, for example, parallel, formed of an inductance L2in parallel with a capacitor C2 between two terminals 21 and 22. Theparallel oscillating circuit (called receive mode resonant circuit) isintended to capture the magnetic field generated by oscillating circuitL1-C1 of terminal 1. Circuits L2-C2 and L1-C1 are tuned to a sameresonance frequency (for example, 13.56 MHz). Terminals 21 and 22 areconnected to two A.C. input terminals of a rectifying bridge 23 (mostoften, fullwave). The rectified output terminals of bridge 23respectively define a positive terminal 24 and a reference terminal 25.A capacitor Ca is connected between terminals 24 and 25 to smooth therectified voltage. The recovered power is used to recharge a battery,not shown.

When transponder 2 is in the field of terminal 1, a high-frequencyvoltage is generated across resonant circuit L2-C2. This voltage,rectified by bridge 23 and smoothed by capacitor Ca, provides a supplyvoltage to electronic circuits of the transponder via a voltageregulator 26 (REG). Such circuits generally comprise a processing unit27 (for example, a microcontroller μC) associated with a memory (notshown), a demodulator 28 (DEM) of the signals that may have beenreceived from terminal 1, and a modulator 29 (MOD) for transmitting datato the terminal. The transponder is generally synchronized by means of aclock (CLK) extracted, by a block 20, from the high-frequency signalrecovered, before rectification, from one of terminals 21 and 22. Mostoften, all the electronic circuits of transponder 2 are integrated in asame chip.

To transmit data from terminal 1 to the transponder, circuit 16modulates (generally in amplitude) the carrier (signal OSC) according tosignal Tx. On the side of transponder 2, these data are demodulated bydemodulator 28 based on voltage V_(Ca). The demodulator may sample thesignal to be demodulated upstream of the rectifying bridge.

To transmit data from transponder 2 to terminal 1, modulator 29 controlsa stage 30 of modulation (retromodulation) of the load formed by thetransponder circuits on the magnetic field generated by the terminal.This stage is generally formed of an electronic switch K30 (for example,a transistor) and of a resistor R30 (or a capacitor), in series betweenterminals 24 and 25. Switch K30 is controlled at a so-called sub-carrierfrequency (for example, 847.5 kHz), much lower (generally with a ratioof at least 10) than the frequency of the excitation signal of theoscillating circuit of terminal 1. When switch K30 is on, theoscillating circuit of the transponder is submitted to an additionaldamping with respect to the load formed by circuits 20, 26, 27, 28, and29 so that the transponder samples a greater amount of power from thehigh-frequency magnetic field. On the side of terminal 1, amplifier 14maintains the amplitude of the high-frequency excitation signalconstant. Accordingly, the power variation of the transponder translatesas an amplitude and phase variation of the current in antenna L1. Thisvariation is detected by an amplitude or phase demodulator of theterminal. In the embodiment illustrated in FIG. 2, comparator 17integrates a phase demodulator also used to demodulate the signaloriginating from the transponder. Accordingly, comparator 17 provides asignal Rx giving back to circuit 11 a possible retromodulation of datareceived from a transponder. Other demodulation circuits may beprovided, for example, a circuit exploiting a measurement of the voltageacross capacitor C1.

Many variations exist to encode/decode and modulate/demodulatecommunications between a transponder and a terminal.

The response time of the phase regulation loop is sufficiently long toavoid disturbing the possible retromodulation from a transponder andsufficiently short as compared with the speed at which a transponderpasses in the field of the terminal. One can speak of a staticregulation with respect to the modulation frequencies (for example, the13.56-MHz frequency of the remote supply carrier and the 847.5-kHzretromodulation frequency used to transmit data from the transponder tothe terminal).

An example of a phase regulation terminal is described in documentEP-A-0857981.

Regulating the phase on the terminal side enables using current andvoltage measurements in the oscillating circuit of the transponder todeduce from these measurements information relative to the transpondercoupling when it is in the field of the terminal. The couplingcoefficient between the oscillating circuit of the terminal and of thetransponder essentially depends on the distance separating thetransponder from the terminal. The coupling coefficient, noted k, isalways between 0 and 1. It can be defined by the following formula:

$\begin{matrix}{{k = \frac{M}{\sqrt{L\; {1 \cdot L}\; 2}}},} & \left( {{formula}\mspace{14mu} 1} \right)\end{matrix}$

where M represents the mutual inductance between inductances L1 and L2of the oscillating circuits of the terminal and of the transponder.

An optimum coupling is defined as being the position at which voltageV_(C2) across the oscillating circuit of the transponder is maximum.This optimum coupling, noted k_(opt), may be expressed as:

$\begin{matrix}{{k_{opt} = \sqrt{\frac{L\; 2}{L\; 1} \cdot \frac{R\; 1}{R\; 2}}},} & \left( {{formula}\mspace{14mu} 2} \right)\end{matrix}$

where R2 represents the resistance equivalent to the load formed by theelements of the transponder on its own oscillating circuit. In otherwords, resistance R2 represents the equivalent resistance of all thecircuits of transponder 2, placed in parallel on capacitor C2 andinductance L2 (before or after the rectifying bridge). The conductancedue to the transponder circuits will be called “resistive load”. Thelevel of this load is symbolized by resistor R2 in parallel across theoscillating circuit. In above formula 2, the series resistance ofinductance L1 (terminal antenna) has been neglected. It can also beconsidered that the value of this series resistance is, forsimplification, included in the value of resistor R1.

Formula 2 represents a signature of the terminal-transponder couple. Forthe same transponder and given operating conditions (load R2), theoptimum coupling coefficient varies according to the terminal whichconditions values L1 and R1.

It is provided to take advantage of this feature to enable a transponderto authenticate the terminal in the range of which it is located byindirectly verifying this signature and, similarly, to enable theterminal to authenticate the transponder.

To authenticate the terminal-transponder couple, the value of voltageV_(C2) across capacitive element C2 of its oscillating circuit isexploited. This voltage is provided by the following relation:

$\begin{matrix}{{V_{C\; 2} = \frac{I\; 2}{\omega \cdot C_{2}}},} & \left( {{formula}\mspace{14mu} 3} \right)\end{matrix}$

where I2 represents the current in the oscillating circuit of thetransponder, and where ω represents the pulse of the signal.

Current I2 is equal to:

$\begin{matrix}{{{I\; 2} = \frac{{M \cdot \omega \cdot I}\; 1}{Z\; 2}},} & \left( {{formula}\mspace{14mu} 4} \right)\end{matrix}$

where I1 represents the current in the oscillating circuit of theterminal and where Z2 represents the transponder impedance.

Impedance Z2 of the transponder is provided by the following relation:

$\begin{matrix}{{{Z\; 2^{2}} = {{X\; 2^{2}} + \left( \frac{L\; 2}{R\; {2 \cdot C}\; 2} \right)^{2}}},} & \left( {{formula}\mspace{14mu} 5} \right)\end{matrix}$

where X2 represents the imaginary part of the impedance of theoscillating circuit

$\left( {{X\; 2} = {{{\omega \cdot L}\; 2} - \frac{1}{{\omega \cdot C}\; 2}}} \right)$

and where R2 represents the resistance equivalent to the load formed bythe transponder elements on its own oscillating circuit. In other words,resistance R2 represents the equivalent resistance of all the circuits(microprocessors, retromodulation means, etc.) of transponder 2, broughtin parallel on capacitor C2 and inductance L2 (before or after therectifying bridge). The conductance due to the transponder circuits, andthus their consumption, will be called “resistive load”. The level ofthis load is symbolized by resistor R2 in parallel across theoscillating circuit.

Further, current I1 in the oscillating circuit of the terminal is givenby the following relation:

$\begin{matrix}{{{I\; 1} = \frac{Vg}{Z\; 1_{app}}},} & \left( {{formula}\mspace{14mu} 6} \right)\end{matrix}$

where Vg designates a so-called generator voltage, exciting theoscillating circuit of the terminal, and where Z1 _(app) represents theapparent impedance of the oscillating circuit.

Regulating the phase of the oscillating circuit of the terminal enablesall the variations which would tend to modify, statically with respectto the modulation frequencies, the imaginary part of the load formed bythe transponder, to be compensated by the phase regulation loop. It isthus ensured that in static operation, the imaginary part of impedanceZ1 _(app) is zero. Accordingly, impedance Z1 _(app) becomes equal toapparent resistance R1 _(app) (real part of the impedance) and may beexpressed as:

$\begin{matrix}{{Z\; 1_{app}} = {{R\; 1_{app}} = {{R\; 1} + {\frac{{k^{2} \cdot \omega^{2} \cdot L}\; {1 \cdot L}\; 2^{2}}{Z\; {2^{2} \cdot R}\; {2 \cdot C}\; 2}.}}}} & \left( {{formula}\mspace{14mu} 7} \right)\end{matrix}$

In above formula 7, the series resistance of inductance L1 (terminal ofthe antenna) has been neglected. It can also be considered that thevalue of this series resistance is, for simplification, included in thevalue of resistance R1.

Since the oscillating circuits are tuned, it can be considered thatimaginary part X2 of impedance Z2 is, as a first approximation, close tozero. As a result, the value of impedance Z2 can be written as:

$\begin{matrix}{{Z\; 2} = {\frac{L\; 2}{R\; {2 \cdot C}\; 2}.}} & \left( {{formula}\mspace{14mu} 8} \right)\end{matrix}$

By inserting this simplification into formulas 4 and 7, and insertingformula 4 into formula 3, the following formula can be obtained forvoltage V_(C2) recovered across the oscillating circuit of thetransponder:

$\begin{matrix}{V_{C\; 2} = {k \cdot \sqrt{\frac{L\; 1}{L\; 2}} \cdot {\frac{V\; g}{\frac{R\; 1}{R\; 2} + {k^{2} \cdot \frac{L\; 1}{L\; 2}}}.}}} & \left( {{formula}\mspace{14mu} 9} \right)\end{matrix}$

Formula 9 shows that, for a given terminal (fixed values of Vg, R1, andL1) and for a fixed impedance L2 (and thus a fixed value of C2), voltageV_(C2) only depends on coupling k and on the resistive load (equivalentto resistor R2) formed by the transponder circuits and brought inparallel on the oscillating circuit.

It should be noted that formula 9 can only be applied when theoscillating circuit of transponder L2-C2 is considered to be set to thetuning frequency, that is, ω·√{square root over (L2·C2)}=1.

For a given coupling value k, considering that the impedance of theoscillating circuit of the terminal does not vary and that the circuitsremain tuned, the ratio between values V_(C2]R21) and V_(C2]R20) ofvoltage V_(C2), respectively for values R21 and R20 of resistor R2,provides, according to formula 2 and 9, the following relation:

$\begin{matrix}{\frac{V_{{{C\; 2}\rbrack}R\; 21}}{V_{{{C\; 2}\rbrack}R\; 20}} = {\frac{\left( \frac{k}{k_{{{opt}\rbrack}\; R\; 20}} \right)^{2} + 1}{\left( \frac{k}{k_{{{opt}\rbrack}R\; 20}} \right)^{2} + \frac{R\; 20}{R\; 21}}.}} & \left( {{formula}\mspace{14mu} 10} \right)\end{matrix}$

Formula 10 shows that by increasing the value of resistor R2 from afirst value R20 to a second greater value R21 (which amounts todecreasing the load of the transponder circuits on oscillating circuitL2-C2), voltage V_(C2]R21) will be greater than voltage V_(C2]R20).Conversely, a decrease in the value of transistor R2 causes a decreasein recovered voltage V_(C2).

Another characteristic operating condition of the terminal-transpondercouple is linked to an off-load operation of the terminal.

Formulas 6 and 7 enable to write:

$\begin{matrix}{{I\; 1} = {\frac{Vg}{{R\; 1} + {{k^{2} \cdot \frac{L\; 1}{L\; 2} \cdot R}\; 2}}.}} & \left( {{formula}\mspace{14mu} 11} \right)\end{matrix}$

The off-load values represent the current and the voltage on theterminal side when no transponder is present in the field of theterminal. In this off-load operation, the apparent impedance of theoscillating circuit of the terminal now only depends on its componentsR1, C1, and L1. Further, due to the phase regulation, the imaginary partof this impedance is always zero. Formula 11 becomes:

$\begin{matrix}{\; {{I\; 1_{{off} - {load}}} = {\frac{Vg}{R\; 1}.}}} & \left( {{formula}\mspace{14mu} 12} \right)\end{matrix}$

Formulas 11 and 12 enable to write that, for a same current coupling k:

$\begin{matrix}{k^{2} = {\frac{R\; 1}{R\; 2} \cdot \frac{L\; 2}{L\; 1} \cdot {\left( {\frac{I\; 1_{{off} - {load}}}{I\; 1} - 1} \right).}}} & \left( {{formula}\mspace{14mu} 13} \right)\end{matrix}$

The combination of formulas 12 and 13 provides:

$\begin{matrix}{\left( \frac{k}{k_{opt}} \right)^{2} = {\frac{I\; 1_{{off} - {load}}}{I\; 1} - 1.}} & \left( {{formula}\mspace{14mu} 14} \right)\end{matrix}$

The current ratios thus provide information about the optimum couplingcoefficient, and thus about the system signature for a given load.

Further, when a transponder is present in the field of the terminal witha given resistive load (for example, equivalent to a resistor R2 ofvalue R20), the terminal can measure the value of current I1 _(]R20) inits oscillating circuit L1-C1.

When the ratio of the recovered voltages with two values R20 and R21 ofresistor R2 is expressed, for a given coupling k, and combining formulas10 and 14, the following relation is obtained:

$\begin{matrix}{\frac{V_{{{C\; 2}\rbrack}R\; 21}}{V_{{{C\; 2}\rbrack}R\; 20}} = {\frac{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}}}{\frac{R\; 20}{R\; 21} + \left( {\frac{I_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}} - 1} \right)}.}} & \left( {{formula}\mspace{14mu} 15} \right)\end{matrix}$

This relation may also be written, for R20<R21, as:

$\begin{matrix}{{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}} = \frac{1 - \frac{R\; 20}{R\; 21}}{1 - \frac{V_{{{C\; 2}\rbrack}R\; 20}}{V_{{{C\; 2}\rbrack}R\; 21}}}},} & \left( {{formula}\mspace{14mu} 16} \right)\end{matrix}$

or, for R20>R21, as:

$\begin{matrix}{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}} = {\frac{\frac{R\; 20}{R\; 21} - 1}{\frac{V_{{{C\; 2}\rbrack}R\; 20}}{V_{{{C\; 2}\rbrack}R\; 21}} - 1}.}} & \left( {{formula}\mspace{14mu} 16^{\prime}} \right)\end{matrix}$

When ratio R21/R20 is expressed based on formula 16, the followingrelation is obtained:

$\begin{matrix}{{\frac{R\; 20}{R\; 21} = \frac{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 21}} - 1}{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}} - 1}},} & \left( {{formula}\mspace{14mu} 17} \right)\end{matrix}$

whether value R20 is smaller or greater than value R21.

Relation 17 may also be expressed as:

$\begin{matrix}{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 21}} = {{\frac{R\; 21}{R\; 20} \cdot \left( {\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}} - 1} \right)} + 1.}} & \left( {{formula}\mspace{14mu} 18} \right)\end{matrix}$

It is provided to use these ratios between off-load currents and under agiven resistive load (equivalent to a resistor R2 of value R20 or R21)to have the terminal authenticate the transponder and to have thetransponder authenticate the terminal.

FIG. 3 is a functional block diagram of an embodiment of a procedure ofmutual authentication of a terminal and of a transponder.

The off-load current in the oscillating circuit of the terminal isassumed to have been previously stored and recorded. Preferably, thisoff-load current determination is performed while the terminal is in itsfunctional environment to take into account possible static disturbancesinfluencing the measurement. According to a preferred variation, thevalue of the off-load current is periodically updated (for example, itis programmed to be measured in off-load periods of the system when itis known that no transponder is present).

When the terminal detects a transponder in the field, it measures (block41, MES I1 _(]R20)) current I1 in its oscillating circuit (for example,by means of element 15), then calculates (block 42, CALC (I1_(off-load)/I1 _(]R20))_(MES)) the ratio between the measured value andthe off-load current. The result is transmitted to the transponder,which stores it (block 52, STORE (I1 _(off-load)/I1 _(]R20))_(MES)).

The transponder measures and stores (block 51, MES V_(C2]R20)), beforeor after having received the information relative to the current fromthe terminal, voltage V_(C2) across capacitor C2 with a first value R20of resistor R2.

Then (block 53, R20->R21), it modifies its resistive load so thatequivalent resistor R2 takes a value R21. Value R21 is for exampleselected to be greater than value R20. The transponder then measures(block 54, MES V_(C2]R21)) voltage V_(C2) with this value R21 and storesthe result.

The transponder then calculates (block 55, CALC (I1 _(off-load)/I1_(]R20))_(EVAL)) an expected value of the ratio between the values ofcurrents I1 of the off-load terminal and of the terminal with resistorR20, and compares (block 56, (I1 _(off-load)/I1 _(]R20))_(MES)=(I1_(off-load)/I1 _(]R20))_(EVAL)?) the evaluated value with the measuredvalue received from the terminal.

In case of a divergence (output N of block 56), this means that theoscillating circuit of the terminal does not respect the signature.

If test 56 confirms identical values, the transponder considers theterminal as authentic (block 58, OK). Otherwise (output N of block 56),it starts an error processing (block 59, ERROR). This processing forexample corresponds to a transaction denial, to a transponder reset, toa fail-soft operation (without performing the functions which arecritical as to the manipulated information), etc. It may also beprovided for the transponder to send messages to mislead or confuse theterminal with intentionally incorrect information, for example, messagescomprising random data. Various other processings may be envisaged, forexample, any error processing usually provided in the absence of anauthentication by a ciphering mechanism.

If the terminal is considered as authentic, the transponder calculatesand transmits to the terminal (block 57, CALC (I1 _(off-load)/I1_(]R21))_(EVAL)) an evaluated value of the ratio between currents I1, inthe off-load state and with value R21.

Simultaneously (at any time after step 53), the transponder notifies theterminal that it has switched its resistive load to another value tocause a new measurement of current I1.

The terminal (block 43, MES I1 _(]R21)) measures current I1, thencalculates (block 44, CALC (I1 _(off-load)/I2 _(]R21))_(MES)) the ratioof off-load current I1 to current I1 _(]R21) and stores the result.

The terminal then verifies (block 45, (I1 _(off-load)/I1 _(off-load)/I1_(]R21))_(MES)=(I1 _(off-load)/I1 _(]R21))_(EVAL)?) the value that ithas measured against the value evaluated by the transponder. In case ofan identity (output Y of block 45), the terminal considers thetransponder as authentic (block 46, OK). Otherwise, it starts an errorprocessing (block 47, ERROR). Like for the transponder, different errorprocessings may be envisaged according to the application (for example,a blocking, the sending of intentionally incorrect information, etc.).

FIG. 4 partially illustrates a variation of FIG. 3 in which calculation55 and test 56 are replaced with an evaluation (block 55′, CALC(V_(C2]R21))_(EVAL)) of voltage V_(C2) with value R21 of resistor R2,and a comparison (block 56′, (V_(C2]R21))_(EVAL)=(V_(C2]R21))_(MES)?) ofthis evaluated value with respect to the value measured at step 54. Therest is identical to the embodiment of FIG. 3. The variation of FIG. 4may be combined with the embodiment of FIG. 3.

Accordingly, for a given terminal (fixed values of Vg and R1) and asystem in the tuned state, the transponder-terminal couple can beauthenticated by exploitation of the voltages obtained on thetransponder side with two resistive loads (equivalent to resistors R20and R21) and of the corresponding currents in the oscillating circuit ofthe terminal.

In practice, voltage V_(C2) is not directly measured across theoscillating circuit of the transponder, but the smoothed voltage acrossvoltage V_(Ca) at the output of rectifying bridge 23. Voltage V_(Ca) isproportional to voltage V_(C2). Since voltage ratios are evaluated, itis not necessary to know the proportionality factor between voltagesV_(C2) and V_(Ca). In a specific embodiment, the measurement isperformed by the microprocessor. The storage of the values of themeasured voltages is performed either in analog fashion or,preferentially in digital fashion over several bits, the number of whichdepends on the desired analysis accuracy.

The tests may be performed in an order different from that indicatedhereabove. However, they are preferentially performed in an order ofincreasing calculation complexity, which enables to more rapidly rejecta terminal which is not adapted to the transponder.

Further, different intermediary values may be stored to be reused in thesuccessive tests or, conversely, calculated on the fly.

A minimum value may be taken into account to decrease the value ofresistor R2, this value corresponding to the minimum acceptable value topreserve a sufficient supply voltage for the transponder circuits. Thisvalue is obtained by considering ratio R20/R2min according to formula16′. Noting V_(C2min) the voltage with resistance R2min, this formulabecomes:

$\begin{matrix}{\frac{I\; 1_{{off} - {load}}}{I\; 1_{\rbrack R\; 20}} = {\frac{\frac{R\; 20}{R\; 2\; \min} - 1}{\frac{V_{{{C\; 2}\rbrack}R\; 20}}{V_{C\; 2\; m\; i\; n}} - 1}.}} & \left( {{formula}\mspace{14mu} 19} \right)\end{matrix}$

Tolerances or acceptable ranges of values may be introduced into thetests to take into account possible operating drifts of the terminal or,in the case of a category of authorized terminals, possible acceptabledispersions among the terminals of this category.

It is thus possible, based on two voltages measurements with tworesistance values of the oscillating circuit of the transponder, toauthenticate the terminal.

Further, the terminal may authenticate the transponder based on twocurrent measurements in its oscillating circuit with these tworesistance values.

These authentications may be exploited by the terminal, by thetransponder, or by both.

FIG. 5 is a block diagram of an embodiment of a transponder 2, equippedto automatically determine, when it is in the field of a terminal (notshown), whether this terminal is authorized. The representation of FIG.5 is simplified with respect to that of FIG. 2. In particular, the meansof demodulation, retromodulation, and for obtaining the clock frequencyhave not been illustrated.

As previously, transponder 2 is based on a parallel oscillating circuitL2-C2 having its terminals 21 and 22 connected to the input terminals ofa rectifying bridge 23. An element for measuring the current Ic intendedfor the processing unit may be provided at the output of regulator 26.Further, a switchable resistive circuit 40 is provided between terminals24 and 25 of rectifying bridge 23. For example, two resistors R43 andR45 are connected in parallel, each being in series with a switch K43,respectively K45. Switches K43 and K45 (for example, MOS transistors)are intended to be switched to implement the method for determining thecoupling position. Processing unit 27 (PU) receives information aboutvoltage V_(Ca) on an input MES to implement the above-described method.In the example of FIG. 5, when the two resistors R43 and R45 arefunctionally connected, resistor R2 (load of the transponder circuits)has value R20. The disconnection of one of the resistors (for example,resistor R43) increases resistance R2 towards value R21. Otherconnections and switchings may be provided according to the embodimentof the implemented method. For example, a single switchable resistor maybe used, considering that one of the two values of resistor R2corresponds to the resistive load of the other transponder circuits.

According to a preferred embodiment, the switchable resistor correspondsto that used for a resistive retromodulation. For example, a firstmeasurement is performed by switching the retromodulation resistor sothat it is functionally in the circuit (switch K30 in the on state inthe example of FIG. 2). Voltage V_(C2]R20) is measured. Then, switch K30is turned off and voltage V_(C2]R21) is measured.

As a variation, the increase or the decrease of equivalent resistance R2is caused by a variation of the power consumption of the transpondercircuits, typically of processing unit 27. For example, to decrease thevalue of resistor R2 (increase the power consumption), the execution ofcalculations or of processings by unit 27 is triggered. An increase ofequivalent resistance R2 may also be caused by decreasing theconsumption of unit 27 by interrupting certain calculations. As avariation, the execution speed conditioned by the clock is slowed down(block 20). The variation of resistance R2 is known from the time whenthe power consumption of different tasks to be executed by unit 27 isknown.

The calculations required to authenticate a terminal are sufficientlysimple for their execution time to be negligible with respect to thedisplacement speed of a transponder in front of a terminal (and thus thevariation speed of the coupling coefficient). Such is in particular thecase for transponders equipped with microcontrollers executingcryptography functions in which these calculation-intensive functionsare themselves executed in a duration for which it can be consideredthat the coupling does not vary. In other cases, the transponder remainslaid on a reception surface of the terminal and the coupling thus doesnot vary for an even longer period.

It should be noted that if a hacker attempts to intercept the exchangedvalues during the authentication, its simple presence in the fieldmodifies the impedances seen by the terminal and/or the transponder andcauses a failure of the authentication.

It should be noted that the authentication is performed by simplecalculations and measurements.

Various embodiments with different variations have been describedhereabove. It should be noted that those skilled in the art can combinevarious elements of these various embodiments and variations withoutshowing any inventive step. In particular, the selection and the orderof the tests to be performed depend on the application, for example, onthe time available to perform the authentication, on the calculatingcapacity of the transponder, etc.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

1. A method of authentication, by a terminal generating a magneticfield, of a transponder located in this field, wherein: first data,relative to the current in an oscillating circuit of the terminal,measured by the terminal for a first value of the resistive load of thetransponder, are transmitted to the transponder; second correspondingdata are evaluated by the transponder for a second value of theresistive load and are transmitted to the terminal; and said second dataare compared with third corresponding data, measured by the terminal forthe second value of the resistive load.
 2. The method of claim 1,wherein the transponder evaluates said second data based on said firstdata and on fourth data relative to the level of a D.C. voltagegenerated by an oscillating circuit of the transponder, respectivelymeasured for said first value of the resistive load and for a secondresistive load value.
 3. The method of claim 1, wherein said data areratios of the current in the oscillating circuit of the terminal whileno transponder is located in its field and of this same current with thevalues of the resistive load.
 4. The method of claim 1, wherein, in theabsence of an authentication, the terminal sends intentionally incorrectdata.
 5. A method for authenticating a terminal generating a magneticfield and a transponder which is present in its field, wherein: thetransponder is authenticated by the terminal of claim 1; and toauthenticate the terminal, the transponder exploits said first andfourth data.
 6. The method of claim 5, wherein the transponder:evaluates, based on said first and fourth data, a ratio between valuesof the current in the oscillating circuit of the terminal; and comparesthis ratio with said first data.
 7. The method of claim 5, wherein thetransponder: evaluates, based on said first and fourth data, a value ofsaid voltage; and compares this evaluated value with the measured value.8. The method of claim 6, wherein, in the absence of an authenticationby the transponder, said transponder sends intentionally incorrect data.9. An electromagnetic transponder comprising: an oscillating circuitupstream of a rectifying circuit capable of providing a D.C. voltagewhen the transponder is in the magnetic field of a terminal; and atleast one processing unit capable of implementing the method of claim 1.10. A terminal capable of generating an electromagnetic field for atransponder, comprising means capable of implementing the method ofclaim 1.